During the current funded period of this research, we have successfully implemented MRI-based thermometry for the monitoring of focused ultrasound treatments. We have used MRI-based thermometry to determine the threshold thermal dose for tissue damage during short sonications, and we have demonstrated its usefulness in a clinical setting in treatments of breast fibroadenomas, breast cancer, and uterine fibroids. We have also developed methods for treatment planning and shown the impact of wavefront distortion by overlying tissues. Based on these results, we propose to further this research by testing the following hypothesis: Intracavitary ultrasound phased arrays can be used to effectively coagulate prostate tumors when MRI temperature monitoring is used. To test this hypothesis we will first, explore the feasibility of using intracavitary focused ultrasound phased arrays with MRI monitoring by developing a linear array system for minimally invasive ultrasound surgery of early stage prostate cancer and performing pre-clinical tests of it in an animal model. Second, we will explore methods to use MRI derived temperature information to control the treatment so that the coagulation of the complete target volume can be accomplished. Finally, we will develop full-scale two-dimensional transrectal arrays that will exploit the full potential of phased array ultrasound technology for the treatment of prostate disease. At the close of this research we will have developed the necessary methods for clinical feasibility testing of a precise and monitored thermal coagulation method for prostate cancer that can reduce both the treatment time and the impotency rate. This procedure may replace current open surgery with a method that could potentially be performed on outpatient basis and is much less invasive. The healthcare benefits of the proposed treatments are significant and include faster recovery, reduced cost, and a lowered risk of infection and bleeding